Fig. 2.

Combined impact of sleep, the circadian rhythm and associated release of cortisol and epinephrine on rhythms and redistribution of leukocyte subsets. Sleep compared with nocturnal wakefulness enhances the homing of naïve T helper (Th) cells to lymph nodes which leads to slightly reduced numbers of these cells circulating in blood during sleep. The mechanisms of this enhanced homing of cells during sleep are not understood. During daytime wakefulness, the circadian rise in cortisol induces an increase in CXCR4 expression on undifferentiated or less differentiated leukocytes, like naïve Th cells, which in turn enables the redistribution of these cells to the bone marrow. On the other hand, epinephrine controls the rhythm of highly differentiated leukocytes, like cytotoxic natural killer (NK) cells, acting as effector cells. During daytime wakefulness, the enhanced activation of β2-adrenoceptors by epinephrine attenuates CX3CR1/CD11a signalling, which leads to an enhanced mobilisation of theses cells from the marginal pool during daytime. Reduced epinephrine levels during sleep (compared to nocturnal wakefulness) allow the margination of these cells, which results in lower cell numbers in peripheral blood. Modified from Lange and Born [71]